Ultrapure Graphene Is a Poor Electrocatalyst: Definitive Proof of the Key Role of Metallic Impurities in Graphene-Based Electrocatalysis.

Graphene and its derivatives have been reported in many articles as "metal-free" carbon electrocatalytic materials. Its synthesis procedures are generally based on the chemical oxidation of graphite and subsequent thermal or chemical reduction. Because graphene oxide has a large surface area and typically contains a variety of oxygen functionalities, metallic ions (impurities) from reaction mixtures can be adsorbed on its surface. These impurities can significantly enhance the electrocatalytic activity and thus lead to data misinterpretation; such impure samples are referred to as "metal-free" catalysts. In this paper, we report the synthesis of impurity-free graphene, which is compared with graphene prepared by standard methods based on the thermal and chemical reduction of two graphene oxides. Detailed analysis of graphene prepared by standard methods shows a direct relation between metallic impurities and the electrocatalytic activity of graphene. In contrast, impurity-free graphene exhibits poor electrocatalytic activity.